1. Field of the Invention
The present invention relates to a copper alloy sheet for forming electronic devices, such as semiconductor devices and, more particularly to a copper alloy sheet for forming QFN packages that need to be processed by dicing and a QFN package having a lead frame made from the copper alloy sheet.
2. Description of the Related Art
Recently, QFN packages have become prevalently used and have progressively replaced packages provided with external leads represented by QFPs and leadless packages, such as CSPs and BGAs.
QFNs have become prevalently used not only because the existing QFP production lines can be used for manufacturing QFN packages, but also because QFN packages have a small area as well as BGAs and QFN packages can be formed in a small thickness comparable to that of CSPs. The QFN package having a metal lead frame can be produced at a high productivity and is superior to BGA in reliability. Since a heat spreader for heat dissipation can be formed on the back surface of the QFN package, any heat sink like that needed by BGAs and CSPs is unnecessary and hence the QFN package can be manufactured at a low cost.
To assemble the QFN package efficiently, lead frame patterns are arranged in a 5×5 or 10×10 matrix, chips are mounted on the lead frames, packages are formed simultaneously by molding a resin after the completion of a wire bonding process. Then the QFN packages are divided into individual QFN packages by dicing. Such a technique is disclosed in JP-A 2003-347494, JP-A 2003-124420 and JP-A 2007-123327. Originally, the dicing process is a process using a very thin dicing blade coated with diamond dust for cutting a wafer into Si chips. This dicing process is applied to cutting the resin molding having the lead frame patterns arranged in a 5×5 or 10×10 matrix into individual QFN packages.
In a QFN package assembling procedure including the dicing process, external leads of the lead frame are cut in the dicing process. Therefore, the lead frame needs to be formed of a material excellent in dicing workability.
Formation of burrs in the external leads cut by the dicing blade, which are called lead burrs, and the abrasion of the dicing blade are problems in the dicing process. Long burrs formed on the cut leads make stable soldering for mounting packages on a substrate impossible, reduce the productivity of a mounting process and deteriorates the reliability of the packages. The rapid abrasion of the dicing blade increases dressing frequency, and the use of a greatly worn out dicing blade produces packages of sizes different from a desired size, and reduces productivity and the yield of packages. Thus, a material for forming a lead frame for QFN packages needs to be capable of reducing lead burrs to the least possible extent and of suppressing the abrasion of the dicing blade as effectively as possible.
Conventional QFP lead frames disclosed in JP-A 2003-124420 and JP-A 2007-123327 are made of copper, a copper alloy and a Fe—Ni alloy, respectively. The copper alloy is an alloy designated by CDA194 containing Cu as a principal component, 2.3% by mass Fe, 0.03% by mass P and 0.13% by mass Zn. The alloy CDA194 is unsatisfactory in satisfactorily reducing lead burrs and in suppressing the abrasion of the dicing blade. Thus, a lead frame material still more excellent in dicing workability has been desired.